In emission regulations of diesel engines, reduction of soot is just as important as reduction of NOx. The DPF is known as an effective technology for this.
The DPF is a diesel particulate filter. While the engine operates at low exhaust temperature, the soot keeps accumulating in the DPF. The accumulated soot is removed by filter regeneration, i.e. by actively raising the temperature to burn the accumulated soot.
When estimation accuracy for estimating an amount of the soot accumulated in the DPF is low, it is hard to determine when to start or end the regeneration. This can lead to over-accumulation of the soot, resulting in adverse affect on the engine. Further, when performing the regeneration in the over-accumulation state, a significant amount of soot burns and the temperature of the DPF becomes too high, which may lead to melting of the DPF. Thus, it is necessary to estimate the accumulation state of the soot with accuracy.
In off-road vehicles such as forklifts and constructions machines, compared to on-road vehicles such as trucks, the engine operation state changes significantly. In the off-road vehicles, exhaust temperature changes significantly and in some cases, fails to meet the conditions necessary for the regeneration during the active regeneration, or the engine is turned off intentionally by an operator. This can causes frequent stopping of the active regeneration.
Therefore, it is necessary to appropriately control conditions such as the timing for resuming the active regeneration after the stopping of the regeneration and the regeneration time. When the regeneration is not resumed at the appropriate timing after the stopping of the regeneration, it could result in frequent regeneration, poor rate of fuel consumption and oil dilution.
Therefore, it is necessary to precisely determine the timings such as when to start or end the active regeneration and when to resume the active regeneration after the stopping of the regeneration. Thus, it is crucial to estimate the accumulation state of the soot with high precision.
Other than from a discharge amount of the soot, the amount of the accumulated soot can be estimated from may other indexes such as a pressure difference between front and back of the DPF, the operation time of the engine (a distance traveled), and a fuel consumption rate.
For instance, disclosed in Patent Literature 1 (JP 4070687 B) is how to determine a timing to resume the regeneration by estimating an amount of PM (Particulate Matter) accumulated in the DPM, based on a calculation value (an estimation value) of PM accumulated in the DPM, a pressure difference between front and back of an exhaust gas purification apparatus, an operation time (a distance traveled), select a regeneration mode based on the determination result and then resume the regeneration. As shown in FIG. 7, regeneration temperatures and regeneration times are set in advance for corresponding regeneration modes.